1. Field of the Invention
This invention relates generally to the field of rotation monitoring and control systems, and more particularly relates to optical encoders for providing information relating to the rotational phase of a rotating disc.
2. Description of Related Art
Shaft angle encoders and rotational angle encoders producing sensor voltages having pulse characteristics or sinusoidal waveform characteristics have been used for controlling shaft angle position in mechanical control systems, and have been used in tachometers for monitoring rotational velocity, or other parameters, by counting either complete or partial revolutions In a device such as fluid pump, the volume of fluid dispensed can be monitored by counting the number of complete or partial revolutions of a cam wheel. Where such a cam wheel or disc contains reflective portions or apertures, the phase of rotation of the wheel or disc may be monitored optically; however, monitoring of rotational position is subject to error due to vibrations, particularly when the optical wheel is subject to slower rotation or stopping.
In the circumstance of connection of a medical infusion pump between an IV bottle and an IV line to a patient, the monitoring of fluid flow may be subject to error due to vibrations from patient movement, or movement of the patient from one location to another. Typically, such medical infusion systems will provide an alarm signal when an error in the monitoring of fluid flow due to vibration is detected, and the infusion is interrupted until the system can be reset in a vibration free situation.
One prior art method of error detection due to vibration is "quadrature" detection, which is a common industry standard. Where an optically encoded disc includes a row of volume counting flags, "quadrature" detection involves the use of an additional row of flags offset from the row of volume counting flags by one quarter of the distance from the beginning of one flag to the beginning of the next flag. By monitoring the two rows of offset counting flags, the system can provide an indication of both the direction and amount of motion, and the system is thereby less subject to errors due to vibration. According to this scheme, if the distance from the leading edge of one flag on one row of flags to the leading edge of the next flag on the same row of flags is considered to be 360 degrees, and the length of the space between the flags on either row is equal to the length of the flags on that row, then a new signal would be encountered every 90 degrees, thus the name "quadrature" detection. Accordingly, implementation of the quadrature detection scheme requires three rows of optical encoder flags, and three optical interrupters, if the system is also to include a row of tachometer flags.
Vibration compensation can also be accomplished electronically. Tachometers including anti-jitter circuitry for monitoring the passage of three sets of slots arranged around a rotating disc are known; a tachometer is also known that includes a rotating disc with apertures and which utilizes electronics, including Schmitt trigger circuitry with a minimum hysteresis of 2 degrees, to ensure against undesired readings due to backlash or vibrations. Various shapes for optical flags on optical discs are also known in the art; and a tachometer utilizing an optical disc with apertures designed to produce pulses having leading and trailing edges is also known. However, vibration-related errors can still occur as the optical disc rotates and as the flags are counted, if the speed of rotation is slow enough and a vibration of the optical disc occurs, so that the disc moves in a reverse direction, corrupting the count. It would be desirable to provide an optical encoder apparatus, which instead of relying upon detection of the edges of an optical flag essentially blurs the edge transistion of an optical flag. It would be desirable to maximize this blurred region in order to obtain a maximum insensitivity to vibration. It would also be desirable to provide an electronic signal processing system designed to identify and monitor intermediate portions of a waveform generated by the optical flags, in order to avoid false readings due to edge transistion vibrational errors.